The present invention provides an accelerated process for preparing a methyl ester by reacting a carboxylic acid or salt thereof with dimethyl carbonate in the presence of a catalyst selected from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4-diazabicyclo[2.2.2]octane (DABCO), and 4-dimethylaminopyridine (DMAP), wherein said reaction is conducted under microwave irradiation at a temperature of from about 120xc2x0 C. to 300xc2x0 C. for a period of time from about 1 second to about 300 minutes.
Methylation of alcohols, amines, carboxylic acids, and activated methylenes is an important process in chemistry. However, due to the environmental and human impact of using toxic and unsafe methylating reagents such as methyl iodide or dimethyl sulfate, the investigation of safer, generally applicable alternatives continues. As an alternative to these toxic methylating agents, dimethyl carbonate has attracted considerable attention for the methylation of phenols, anilines, and activated methylenes. Dimethyl carbonate is non-toxic and generates CO2 and methanol as by-products during methylations. Dimethyl carbonate is also a volatile liquid with a boiling point of 90xc2x0 C. Hence, the unreacted dimethyl carbonate can be easily recovered by distillation from the reaction mixture and reused.
Microwave irradiation has been used for moisture analysis, organic synthesis, acid decomposition of botanical or biological samples, and rapid hydrolysis of peptides and proteins. Microwave irradiation has also been applied to several organic reactions. Krstenansky et al., Curr. Opin. Drug Discovery Dev., Vol. 3, p. 454 (2000), describes converting alkyl and aryl amides to their corresponding nitrites under microwave irradiation. Wang et al., Synthetic Communications, Vol. 26, No. 2, pp. 301-305 (1996), describes using microwave irradiation to synthesize aromatic ethers from phenols and halide in the absence of organic solvent and inorganic carrier. Bogdal et al., Synthetic Communications, Vol. 28, No. 16, pp. 3029-3039 (1998), describes reacting phenols with primary alkyl halides under microwave irradiation to synthesize aromatic ethers.
Elder et al., Journal of Chemical Education, Vol. 73, No. 5, pp. 104-105 (1996), describes using microwave irradiation to synthesize the following compounds: anthracene, octyl acetate, 2-naphthyl acetate, 2-methoxynaphthalene, and n-phenyl-2,4-dinitroaniline. Kabza et al., Journal of Organic Chemistry, Vol. 65, No. 4, pp. 1210-1214 (2000), describes using microwave irradiation in the acid-catalyzed Fisher-type esterification of isopentyl alcohol and acetic acid. Kabza concluded that the esterification reaction behaved comparably under both microwave and thermal conditions. Caddick, Tetrahedron, Vol. 51, No. 38, pp. 10403-10432 (1995), describes microwave assisted organic reactions. Such organic reactions described are: pericyclic, cyclization, aromatic substitution, oxidation, catalytic transfer hydrogenation, alkene functionalization, alkylation, decarboxylation, carbohydrates, radical reactions, protecting groups, condensation, peptide synthesis, silicon, and rearrangement. Perreux et al., Tetrahedron, Vol. 57, pp. 9199-9223 (2001), describes microwave effects in organic synthesis. Such organic syntheses include bimolecular reactions between neutral reactants, bimolecular reactions with one charged reactant, unimolecular reactions.
U.S. Pat. No. 4,513,146 describes a method for producing esters from highly hindered carboxylic acids and carbonates. The method involves reacting the highly hindered carboxylic acid with a carbonate with or without a catalyst at a temperature of 175xc2x0 C. According to the only example, the reaction took 4 hours and 50 minutes. U.S. Pat. No. 4,513,146 states that exemplary bases are nitrogen-containing heterocyclic catalysts such as pyridine, 4-(dimethylamino)pyridine, imidazole, 2,6-lutidine, and 2,4,6-collidine.
U.S. Pat. No. 5,278,333 describes a process for preparing xcex1-phenylmethylpropionate by reacting a mixture of phenylacetic acid, dimethyl carbonate, and potassium carbonate in a molar ratio of 1:20:2, respectively, in an autoclave at a temperature of 225xc2x0 C. for 15 hours.
It would be advantageous from a production standpoint to develop an accelerated process for preparing methyl esters which utilizes dimethyl carbonate as a reactant. In addition, the process should minimize degradation and/or racemization of optically pure compounds, and minimize the formation of by-products.
The invention provides an accelerated process for preparing a methyl ester having formula (III) 
said process comprising reacting a carboxylic acid or salt thereof having formula (I) 
with dimethyl carbonate having formula (II) 
in the presence of a catalyst selected from the group consisting of 1,8-diazabicyclo[5.4.0]undec-7-ene; 1,4-diazabicyclo[2.2.2]octane; 4-dimethylaminopyridine; and combinations thereof, wherein R1 is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl, and haloalkyl; and M is selected from the group consisting of hydrogen, a monovalent metal, and a monovalent fractional part of a polyvalent metal, wherein said process is conducted under microwave irradiation at a frequency from 300 MHz to 30 GHz, and at a temperature of from about 120xc2x0 C. to 300xc2x0 C. for a period of microwave irradiation time from about 1 second to about 300 minutes.
According to another aspect, the invention provides a compound having formula (III) 
wherein said compound is prepared by an accelerated process comprising reacting a carboxylic acid or salt thereof having formula (I) 
with dimethyl carbonate having formula (II) 
in the presence of a catalyst selected from the group consisting of 1,8-diazabicyclo[5.4.0]undec-7-ene; 1,4-diazabicyclo[2.2.2]octane; 4-dimethylaminopyridine; and combinations thereof, wherein R1 is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl, and haloalkyl; and M is selected from the group consisting of hydrogen, a monovalent metal, and a monovalent fractional part of a polyvalent metal, wherein said process is conducted under microwave irradiation at a frequency from 300 MHz to 30 GHz, and at a temperature of from about 120xc2x0 C. to 300xc2x0 C. for a period of microwave irradiation time from about 1 second to about 300 minutes.
The process of the invention is especially advantageous for preparing methyl esters since the process: (1) utilizes an environmentally friendly methylating reagent, dimethylcarbonate; (2) produces a high yield of the methyl ester, generally 95-99% conversion in less than 30 minutes of microwave irradiation; (3) minimizes degradation and/or racemization of optically pure compounds; and (4) minimizes the formation of by-products.
The accelerated process of the invention for preparing a methyl ester utilizes microwave irradiation. The microwave region of the electromagnetic spectrum corresponds to wavelengths from 1 cm to 1 m and frequencies from 300 MHz to 30 GHz. By International Convention, however, domestic and industrial microwave ovens generally operate at greater than 900 MHz, preferably about 2450 MHz to about 2455 MHz, in order to prevent interference with RADAR transmissions and telecommunications. Thus, the entire microwave region is not readily available for heating applications. Sources of microwave irradiation include multimode ovens and monomode ovens which may be batch or continuous devices. A preferred monomode oven is a continuous-flow reactor, such as a Milestone ETHOS-CFR continuous-flow reactor.
The methyl ester has formula (III) 
in formula (III), R1 is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl, and haloalkyl. The reaction is conducted in the presence of a catalyst which is selected from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU); 1,4-diazabicyclo[2.2.2]octane (DABCO); and 4-dimethylaminopyridine (DMAP). A combination of catalysts may also be used.
The process of the invention involves reacting a carboxylic acid or salt thereof having formula (I) 
with dimethyl carbonate having formula (II) 
in formula (I), R1 is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl, and haloalkyl, and M is selected from the group consisting of hydrogen, a monovalent metal, and a monovalent fractional part of a polyvalent metal. The metal is preferably selected from sodium, potassium, magnesium, or calcium.
As used herein, xe2x80x9calkylxe2x80x9d means straight chain or branched alkyl, which may be, for example, C1-C10-alkyl such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, straight or branched pentyl, straight or branched hexyl, straight or branched heptyl, straight or branched nonyl or straight or branched decyl. Preferably alkyl is C1-C4-alkyl.
xe2x80x9cArylxe2x80x9d means C6-C14-aryl, preferably C6-C10-aryl, and may be, for example, substituted by at least one group selected from mercapto, dialkylamino, nitro, alkoxy, halogen, keto, cyano, or a combination. Preferably aryl is phenyl.
xe2x80x9cAlkoxyxe2x80x9d means straight chain or branched alkoxy and may be, for example, C1-C10-alkoxy such as methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, isobutoxy, sec-butoxy, tert-butoxy, or straight or branched pentoxy, hexyloxy, heptyloxy, octyloxy, nonyloxy or decyloxy. Preferably alkoxy is C1-C4-alkoxy.
xe2x80x9cAlkenylxe2x80x9d means straight chain or branched alkenyl, which may be, for example, C2-C10 alkenyl such as vinyl, 1-propenyl, 2-propenyl, 1-butenyl, isobutenyl, or straight or branched pentenyl, hexenyl, heptenyl, octenyl, nonenyl or decenyl. Preferred alkenyl is C2-C4-alkenyl.
xe2x80x9cCycloalkylxe2x80x9d means C3-C10-cycloalkyl having 3- to 8-ring carbon atoms and may be, for example, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl or cycloheptyl, any of which can be substituted by one, two or more C1-C4-alkyl groups, particularly methyl groups. Preferably, cycloalkyl is C3-C6-cycloalkyl.
xe2x80x9cBenzocycloalkylxe2x80x9d means cycloalkyl (e.g., one of the C3-C10-cycloalkyl groups mentioned hereinbefore), attached at two adjacent carbon atoms to a benzene ring. Preferably, benzocycloalkyl is benzo-C5-C6-cycloalkyl, especially benzocyclohexyl (tetrahydronaphthyl).
xe2x80x9cCycloalkylalkylxe2x80x9d means C3-C10-cycloalkyl-C1-C10-alkyl where the C3-C10-cycloalkyl group has 3- to 8-ring carbon atoms and may be, for example, one of the C1-C10-alkyl groups mentioned hereinbefore, particularly one of the C1-C4-alkyl groups, substituted by one of the C3-C10-cycloalkyl groups mentioned hereinbefore. Preferably cycloalkylalkyl is C3-C6-cycloalkyl-C1-C4-alkyl.
xe2x80x9cAralkylxe2x80x9d means straight chain or branched C6-C10-aryl-C1-C10-alkyl and may be, for example, one of the C1-C10-alkyl groups mentioned hereinbefore, particularly one of the C1-C4-alkyl groups, substituted by phenyl, tolyl, xylyl or naphthyl. Preferably, aralkyl is phenyl-C1-C4-alkyl, particularly benzyl or 2-phenylethyl.
xe2x80x9cHeterocyclicxe2x80x9d means a monovalent heterocyclic group having up to 20 carbon atoms and one, two, three or four heteroatoms selected from nitrogen, oxygen and sulfur, the group optionally having an alkyl, alkylcarbonyl, hydroxyalkyl, alkoxyalkyl or aralkyl group attached to a ring carbon or nitrogen atom and being linked to the remainder of the molecule through a ring carbon atom, and may be, for example, a group, preferably a monocyclic group, with one nitrogen, oxygen or sulfur atom such as pyrryl, pyridyl, piperidyl, furyl, tetrahydrofuryl or thienyl, or a group, preferably a monocyclic group, with two hetero atoms selected from nitrogen, oxygen and sulfur, such as imidazolyl, pyrimidinyl, piperazinyl, oxazolyl, isoxazolyl, thiazolyl, morpholinyl or thiomorpholinyl. Preferably, heterocyclic is a monocyclic group having 5- or 6-ring atoms and one or two nitrogen atoms, or one nitrogen atom and one oxygen atom, in the ring and optionally substituted on a ring nitrogen atom by C1-C4-alkyl, hydroxy-C1-C4-alkyl, C1-C4-alkylcarbonyl or phenyl-C1-C4-alkyl.
xe2x80x9cHeteroaralkylxe2x80x9d means straight chain or branched aralkyl (e.g., one of the C6-C10-aryl-C1-C10-alkyl groups mentioned hereinbefore) substituted by one or more heterocyclic groups.
xe2x80x9cAlkoxyalkylxe2x80x9d means straight chain or branched alkyl substituted by one or more alkoxy groups and may be, for example, a C1-C10-alkoxy-C1-C10-alkyl group, such as one of the C1-C10-alkyl groups, particularly one of the C1-C4-alkyl groups, mentioned hereinbefore substituted by one of the C1-C10-alkoxy groups, preferably one of the C1-C4-alkoxy groups, mentioned hereinbefore. Preferably alkoxyalkyl is C1-C4-alkoxy-C1-C4-alkyl.
xe2x80x9cCarboxyalkylxe2x80x9d means straight chain or branched alkyl, for example, C1-C10-alkyl such as one of the C1-C10-alkyl groups mentioned hereinbefore, substituted, preferably on a primary carbon atom, by a carboxyl group. Preferably carboxyalkyl is carboxy-C1-C4-alkyl.
xe2x80x9cAlkylcarbonylxe2x80x9d means a group R2Cxe2x95x90O wherein R2 is alkyl, for example, C1-C10-alkyl such as one of the C1-C10-, preferably C1-C4-, alkyl groups mentioned hereinbefore. Preferably, alkylcarbonyl is C1-C4-alkylcarbonyl, for example, R2Cxe2x95x90O wherein R2 is C1-C4-alkyl.
xe2x80x9cAlkoxycarbonylxe2x80x9d means a group R3CO wherein R3 is an alkoxy group, for example, a C1-C10-alkoxy group such as one of the C1-C10, preferably C1-C4, alkoxy groups mentioned hereinbefore. Preferably, alkoxycarbonyl is C1-C4-alkoxycarbonyl, for example, R3CO wherein R3 is C1-C4-alkoxy.
xe2x80x9cAlkoxycarbonylalkylxe2x80x9d means straight or branched chain alkyl, for example, a C1-C10-alkyl group such as one of the C1-C10-, preferably C1-C4-, alkyl groups mentioned hereinbefore, substituted by an alkoxycarbonyl group as hereinbefore defined. Preferably, alkoxycarbonylalkyl is C1-C4-alkoxy-carbonyl-C1-C4-alkyl.
xe2x80x9cHaloalkylxe2x80x9d means straight chain or branched alkyl, for example, C1-C10-alkyl such as one of the C1-C10-alkyl groups mentioned hereinbefore, substituted by one or more, for example one, two or three, halogen atoms, preferably fluorine or chlorine atoms. Preferably haloalkyl is C1-C4-alkyl substituted by one, two or three fluorine or chlorine atoms.
Specific examples of carboxylic acids of formula (I) are 2,6-dimethoxybenzoic acid, 2,3:4,6-di-O-isopropylidene-xcex1-L-xylo-2-hexulofuranosonate monohydrate, N-xcex1-t-boc-L-proline, benzoic acid, phthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid, diphenyl-4,4xe2x80x2-dicarboxylic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, 2,2-dimethylpropanoic acid, 2,2-dimethylbutanoic acid, 2,2-dimethyloctanoic acid, and the sodium and potassium salts thereof. A combination of carboxylic acids may also be used. Preferably the carboxylic acid is selected from 2,6-dimethoxybenzoic acid, 2,3:4,6-di-O-isopropylidene-xcex1-L-xylo-2-hexulofuranosonate monohydrate, N-xcex1-t-boc-L-proline, and benzoic acid.
The process of the invention is preferably conducted in the liquid phase. It may be carried out in the form selected from batch, continuous, semibatch, or semicontinuous. Dimethyl carbonate is a liquid under the conditions of the reaction and it may act as a solvent for the carboxylic acid or salt thereof. Typically, but not necessarily, excess dimethyl carbonate is employed relative to the amount of carboxylic acid or salt thereof, and this usually serves to dissolve the carboxylic acid or salt thereof throughout the reaction. Although extrinsic solvent is not ordinarily employed, it may be used when desired or when necessary to dissolve one or more of the reactants. Examples of suitable extrinsic solvents include: acetonitrile, ethyl acetate, acetone, tetrahydrofuran, heptane, cyclohexane, t-butyl methyl ether, dimethy sulfoxide, toluene, dioxane, dimethylformamide and chlorinated solvents such as chloroform, methylene chloride, ethylene chloride, and chlorobenzene. A preferred extrinsic solvent is acetonitrile. A combination of solvents may also be used.
The process of the invention is conducted at a temperature of from about 120xc2x0 C. to about 300xc2x0 C., preferably, about 140xc2x0 C. to about 250xc2x0 C.; and more preferably from about 150xc2x0 C. to about 200xc2x0 C. Most preferably, the process is conducted at a temperature of about 160xc2x0 C. The pressure of the reaction depends upon the temperature of the reaction, and the boiling point of the reactants and solvent. The present inventors have determined that a higher reaction temperature may require higher pressure in order to prevent boiling of either the reactants or solvent. Generally, the process is conducted under a pressure of from about 1 bar to about 60 bar, preferably, from about 10 bar to about 35 bar, most preferably about 20 bar.
The process of the invention is conducted for a period of microwave irradiation time sufficient to form a methyl ester. Preferably, the period of microwave irradiation time is from about 1 second to about 300 minutes, more preferably from about 5 minutes to about 30 minutes. Most preferably, the process of the invention is conducted for a period of microwave irradiation time from 6 minutes to 25 minutes.
The equivalent ratio of the catalyst to the carboxylic acid or salt thereof initially present may vary widely, but preferably is in the range of from about 0.01:1 to about 2:1. More preferably, the equivalent ratio of the catalyst to the carboxylic acid or salt thereof initially present is from about 0.1:1 to about 1:1, most preferably, about 1:1.
In one embodiment of the invention, an amine base is used in the process of the invention to prepare a methyl ester. Preferred amine bases are trialkylamines and ethylenediamines. Specific amine bases include, but are not limited to, triethylamine, N,N-diisopropylethylamine and N,Nxe2x80x2-diisopropylethylenediamine. A combination of amine bases may also be used.
Following preparation, the methyl ester may be recovered from the reaction mixture by any of the various techniques known to the art. A preferred method of recovering the methyl ester involves treating the reaction mixture in sequence with an acid solution and a basic solution, followed by distillation of any organic solvent and dimethyl carbonate.